Quantum cascade (QC) lasers are known. See, for instance, U.S. Pat. Nos. 5,457,709 and 5,509,025. See also J. Faist et al., Applied Physics Letters, Vol. 68, p. 3680 (1996), all incorporated herein by reference.
Briefly, a QC laser comprises a multiplicity of identical "repeat units". With each repeat unit is associated an upper and a lower energy level. Under an applied field, charge carriers (typically electrons) migrate from the lower energy level of a given repeat unit to the upper energy level of the adjacent downstream repeat unit, followed by a radiative transition from the upper to the lower level of the repeat unit, then proceeding into the next repeat unit. Thus, each charge carrier that is introduced into the active region of the QC laser ideally undergoes N transitions (N being the number of repeat units, about 25, for instance), each such transition resulting in emission of a photon of wavelength .lambda., typically in the midinfrared (exemplarily 3-13 .mu.m).
U.S. patent application Ser. No. 08/744,292, by Capasso et al. (allowed), incorporated herein by reference, discloses a QC laser wherein a repeat unit comprises a superlattice active region and an injection/relaxation region. The superlattice region provides an upper and a lower miniband, with the radiative transition being from the upper to the lower miniband. After the radiative transition, the charge carrier migrates from the lower miniband through an injection/relaxation region to the upper miniband of the adjacent downstream repeat unit. See also G. Scamarcio et al., Science, Vol. 276, p. 773 (1997), also incorporated herein by reference.
SLQC (superlattice quantum cascade) lasers offer the advantage of wide energy minibands which potentially can carry large current densities without running into level misalignment when the applied voltage is increased. However, prior art SLQC lasers have relatively high optical losses, relatively broad emission linewidths, and reduced population inversion at high temperatures. As a consequence, prior art SLQC lasers typically present larger threshold current densities than standard QC lasers of the same wavelength, such that room temperature operation and/or continuous wave operation of prior art SLQC lasers typically is not possible.
In view of the potentially advantageous characteristics of SLQC lasers, it would be desirable to have available an improved SLQC laser that is substantially free of (or at least less subject to) the above recited shortcomings. This application discloses such an improved SLQC laser.